1. Field of the Invention
This invention relates to a control system for an internal combustion engine installed in a vehicle equipped with an automatic transmission, and more particularly to such a system which lowers the engine output for reducing the shock apt to be felt by the passengers of a vehicle equipped with an automatic transmission at the time of shifting gears.
2. Description of the Prior Art
The passengers of a vehicle equipped with an automatic transmission are apt to experience an unpleasant shock when torque is reapplied to the power train in the course of a transmission gear shift operation. Japanese Laid-open Patent Publication No. 55(1980)-69738 discloses a system for alleviating this shock by lowering engine output during the gear shift operation. A system for this purpose has also been proposed by the assignee and applied for patent under Japanese Patent Application No. 62(1987)-336666 (laid open under the number of 1(1990)-178736, which was filed with the United States and was patented under the number of U.S. Pat. No. 4,889,014).
In the system of this earlier application, the power transmission state of the power transmission means, e.g. the speed ratio between input and output sides of the clutch, is monitored and the engine output is immediately lowered when the clutch slip rate comes to exceed a prescribed limit, irrespective of whether the cause is gear shifting or an abnormality of the clutch. If the increased clutch slip rate is the result of a gear shifting operation, the output of the engine is restored once the state of power transmission has come within prescribed limits set in view of the target gear position so that gear shift shock is avoided and clutch wear is reduced.
However, the reduction in engine output required for protecting the clutch in the case of slip is not necessarily the same as that appropriate for avoiding gear shift shock in either amount or range. The system proposed earlier by the assignee did not take this fact into consideration but lowered the torque by the same amount in both cases. Depending on the driving conditions, therefore, the passengers were likely to experience a degraded driving feeling, specifically a sensation of power deficiency or deceleration.